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Big 40 Runs But does not weld.

Hey guysI have a big 40 serial number: JJ521296 stock number: 903043. Its starts and runs great but when welding there is only a very faint spark when trying to strike. I have read alittle here and elsewhere about these machine and have seen just a few things to do or try but nothing to really let me know what Iam up against.First thing I did was clean the 3 rings (very dirty) and checked the brushes. Also to note I am getting power from the recepticle with grinder. I have checked for loose or burnt wires but nothing jumps out at me. I really need some advice on what Iam possible up against here and some guidance as I know nothing about these machines at all. Can someone please help me out here? Thanks and God Bless!!!

Not alot if any techs on here other than myself, Lets see,,,,,, If you have a remote switch turn it to local, If thats not the case then I'd say that most likely, your local 34 ohm rheostat is pooched

Thanks for the reply!!!. There is not a remote switch on it. But today we tried again and funny thing,,,, after the machine has warmed up and goes to low idle, if you plug a grinder into the outlet, the grinder works fine but as I said the machine does not go to high idle and stays at low idle.. So we put it on high idle and when we pulled the trigger on the grinder there is no longer any power and the grinder will not work? Put it back to low idle and the grinder works??? There must be someone who will know why and what ???? Trying to find out how to check the rheostat. We do have an ohm reader just dont know what to do??? Please help.

Not near the shop, so can't pull up the tech manuals ( can't remember password). In behind the front panel is a big azz rheostat ( big roundish thingy) with a couple wires on it. With an ohm meter check the resistance on it.